Calibration of CD-SEM by e-beam induced current measurement

ABSTRACT

The present invention relates to a system and method for calibrating a scanning electron microscope (SEM). The method comprises measuring an electrical characteristic of a calibration standard reference sample feature via a current induced by an electron beam (e-beam) and correlating the e-beam induced current measurement with an SEM measurement thereof. The correlation of the e-beam induced current and SEM measurements provides a critical dimension (CD) for the reference sample feature which can then be used to correlate SEM measurements of workpiece features. The system provides a reference sample having a measurable feature electrically connected to a probe. The probe provides an electrical measurement of the reference sample feature based on an e-beam induced current. The system further comprises a scanning electron microscope (SEM) adapted to provide an optical measurement of the reference sample feature and workpiece features. A processor is provided to correlate the optical and e-beam induced current measurements of the reference sample feature, whereby a reference feature CD is obtained. The system may further correlate workpiece feature measurements with the reference feature CD in order to determine or obtain a workpiece feature CD.

TECHNICAL FIELD

The present invention generally relates to semiconductor processing and,more particularly, to a system and method for measuring a sample using ascanning electron microscope and calibration thereof.

BACKGROUND OF THE INVENTION

In the semiconductor industry there is a continuing trend toward higherdevice densities. To achieve these high densities there have been, andcontinue to be, efforts toward scaling down the device dimensions onsemiconductor wafers. In order to accomplish such a high device packingdensity, smaller features sizes are required. This may include the widthand spacing of interconnecting lines and the surface geometry such asthe corners and edges of various features.

The requirement of small features with close spacing between adjacentfeatures requires high resolution photo lithographic processes as wellas high resolution inspection and measurement instruments. In general,lithography refers to processes for pattern transfer between variousmedia. It is a technique used for integrated circuit fabrication inwhich, for example, a silicon wafer is coated uniformly with aradiation-sensitive film (e.g., a photoresist), and an exposing source(such as ultraviolet light, x-rays, or an electron beam) illuminatesselected areas of the film surface through an intervening mastertemplate (e.g., a mask or reticle) to generate a particular pattern. Theexposed pattern on the photoresist film is then developed with a solventcalled a developer which makes the exposed pattern either soluble orinsoluble depending on the type of photoresist (i.e., positive ornegative resist). The soluble portions of the resist are then removed,thus leaving a photoresist mask corresponding to the desired pattern onthe silicon wafer for further processing.

In order to control quality in the design and manufacture of highdensity semiconductor devices, it is necessary to measure criticaldimensions (CDs) associated therewith. Semiconductor device featureshaving CDs of interest include, for example, the width of a patternedline, the distance between two lines or devices, and the size of acontact. CDs related to these and other features may be monitored duringproduction and development in order to maintain proper deviceperformance. As device density increases and device sizes decrease, theability to carry out quick, inexpensive, reliable, accurate,high-resolution, non-destructive measurements of CDs in thesemiconductor industry is crucial. The ability to accurately measureparticular features of a semiconductor workpiece allows for adjustmentof manufacturing processes and design modifications in order to producebetter products, reduce defects, etc.

CDs are usually measured during or after lithography. Various operationsperformed during the lithography process may affect the criticaldimensions of a semiconductor device. For example, variations in thethickness of the applied photoresist, lamp intensity during the exposureprocess, and developer concentration all result in variations ofsemiconductor line widths. In addition, line width variations may occurwhenever a line is in the vicinity of a step (a sudden increase intopography). Such topography-related line width variations may be causedby various factors including differences in the energy transferred tothe photoresist at different photoresist thicknesses, light scatteringat the edges of the steps, and standing wave effects. Since thesefactors can greatly affect CDs, fast and reliable monitoring ofsemiconductor device features is important in order to guaranteeacceptable device performance.

Different technologies are currently available to measure CDs associatedwith semiconductor devices. These include: optical microscopy, stylusprofilometry, atomic force microscopy, scanning tunneling microscopy,and scanning electron microscopy. Scanning electron microscopes (SEMs)are commonly used for inspection and metrology in semiconductormanufacturing. The short wavelengths of scanning electron microscopeshave several advantages over conventionally used optical microscopes.For example, scanning electron microscopes may achieve resolutions fromabout 100 to 200 Angstroms, while the resolution of optical microscopesis typically about 2,500 Angstroms. In addition, scanning electronmicroscopes provide depths of field several orders of magnitude greaterthan optical microscopes.

In a typical SEM wafer inspection system, a focused electron beam isscanned from point to point on a specimen surface in a rectangularraster pattern. Accelerating voltage, beam current and spot diameter maybe optimized according to specific applications and specimencompositions. As the scanning electron beam contacts the surface of aspecimen, backscattered and/or secondary electrons are emitted from thespecimen surface. Semiconductor inspection, analysis and metrology isperformed by detecting these backscattered and/or secondary electrons. Apoint by point visual representation of the specimen may be obtained ona CRT screen or other display device as the electron beam controllablyscans a specimen.

Scanning electron microscopes (SEMs) operate by creating a beam ofelectrons accelerated to energies up to several thousand electron volts.The electron beam is focused to a small diameter and scanned across a CDor feature of interest in the scanned specimen. When the electron beamstrikes the surface of the specimen, low energy secondary electrons areemitted. The yield of secondary electrons depends on various factorsincluding the work function of the material, the topography of thesample, the curvature of the surface, and the like. These secondaryelectrons can be employed to distinguish between different materials ona specimen surface since different materials may have significantlydifferent work functions.

Topographic features also affect the yield of secondary electrons.Consequently, changes in height along a specimen surface may be measuredusing an SEM. Electron current resulting from the surface-emittedsecondary electrons is detected and used to control the intensity ofpixels on a monitor or other display device connected to the SEM. Animage of the specimen may be created by synchronously scanning theelectron beam and the display device.

Although SEMs can achieve resolution in the range of angstroms,calibration is difficult. For example, the magnification of an SEM maybe calibrated by placing a sample of known dimensions, such as a chip orwafer having a conductor line of known width, in the instrument andmeasuring the dimension of the sample. The magnification of the SEM isdetermined by dividing the SEM measurement of the image of the sample bythe known dimension of the sample. The magnification calibrationinformation may then be used to construct a calibration curve, or theSEM's magnification controls may be trimmed accordingly.

Calibration according to these prior methods requires samples of knowndimensions. The actual dimensions of a sample, however, may not beprecisely known, or may change. In particular, repeated usage of asingle reference sample as a calibration standard results in degradationof the reference sample. Charge buildup on a reference sample caused byrepeated measurement in a SEM affects the secondary electron emission.Contaminant deposition or buildup also has deleterious effects onmeasurement of a calibration standard reference sample over time.Conventional SEM calibration methods and systems do not account for theerrors in estimating the actual size of a reference feature, and alsofail to account for degradation in reference features.

The measurement of a calibration standard reference sample typicallyinvolves determining where an edge of the sample is. At the sub-micronrange, an edge of a sample may be a complex waveform, as opposed to aflat line. Therefore, in measuring the sample there is uncertainty as toedge location. Where the calibration involves determining the length ofa sample, two edges must be located, and thus the edge determinationuncertainty increases. Further, sample dimensions may vary as a functionof temperature, repeated measurements and electron beam charging causingcontamination. The SEM electron beam may thus cause expansion of areference sample after repeated use. Typically, sample charging duringe-beam exposure or material degradation will broaden or change thesecondary electron signal.

In order to reduce edge determination error, SEM calibration has alsobeen done using a sample having a series of equally spaced lines. Such asample could be a diffraction grating having a plurality of alignedparallel grooves. The SEM is used to measure the pitch of the lines.While this method reduces some of the edge quantification errorsassociated with other SEM calibration methods, higher accuracycalibration methods are needed for SEMs used for measuring high densitysemiconductor devices.

Conventional SEM calibration methods and systems do not account fordegradation of a calibration standard reference sample over time. Forexample, where a line width feature on a reference sample has a knownwidth, repeated scanning of the feature by an SEM results in chargebuildup. This reduces or hampers the ability of an SEM to obtainaccurate measurements of the line width in the future. Contamination ona reference sample feature also prevents or hampers accurate readings.Because conventional SEM calibration methods and systems rely uponaccurate SEM readings of a known reference feature dimension, inaccurateSEM readings of a calibration standard reference feature cause errors inmeasurements of workpiece features performed with the SEM. Inaccuratereadings may lead to unnecessary rework of a product lot therebyincreasing cost.

SUMMARY OF THE INVENTION

The present invention provides a method and system for calibrating ascanning electron microscope, which minimizes or reduces thedisadvantages associated with conventional methods and systems. Inaccordance with one aspect of the present invention, there is provided amethod for calibrating an SEM using an electron beam (e-beam) inducedcurrent measurement of a reference sample dimension. A feature, such asfor example a conductor line on a reference sample, is placed under ane-beam, and a current is induced thereby in the reference feature. Anelectrical characteristic associated with the reference sample featureis then measured. The feature is also measured using SEM techniques. Theelectrical characteristic measurement and the SEM measurement arecorrelated to determine a critical dimension (CD) for the referencesample feature. A workpiece feature is measured using the SEM, and thereference sample feature CD is correlated with the workpiece featuremeasurement in order to obtain a workpiece feature CD.

Because the electrical characteristic measurement is unaffected, oraffected differently, by charge buildup and/or other degradation effectson the reference sample, the correlation between the electrical and SEMreference sample measurements can eliminate or reduce the effects ofthis degradation on system measurements of workpiece features. Theelectrical reference sample measurement, moreover, may provide trendinginformation relating to degradation of the reference sample over time,as well as an indication of the actual size of a reference feature. Inone application of the method, a reference sample is measuredelectrically, and then optically using a SEM. Thereafter, a workpiecefeature is measured, and a workpiece feature CD is obtained using themeasurements of the reference sample for correlation.

In accordance with another aspect of the invention, a method is providedfor calibrating an SEM which comprises providing an electron beam to afeature of a reference sample, whereby a current is induced in thereference sample feature, and measuring an electrical characteristic ofthe reference sample feature to obtain an electrical referencemeasurement. The electrical reference measurement may, for example,comprise measuring or sensing the e-beam induced current through thereference feature, and measuring the voltage across the referencefeature. In this example, the resistance of the reference feature may bedetermined, from which a physical dimension of the reference feature maybe calculated.

The method further comprises measuring the feature of the referencesample using an electron beam to obtain an optical referencemeasurement, and correlating the optical and electrical referencemeasurements to obtain a reference sample feature CD. The electricalmeasurement of the reference feature may provide information as to theactual size of the reference feature, whereas prior methods relied uponestimation of the reference feature size. In addition, the electricalmeasurement may be used to perform trending of degradation of thereference sample feature over time, due to repeated usage in an SEM,charge buildup, corrosion deposition, and the like. Moreover, thecorrelation of the electrical and optical reference feature measurementsmay provide further information about the SEM system through the use ofcalibration magnification or scaling coefficients, stochastics, neuralnetworks, artificial intelligence, data fusion techniques, and/ortrending.

According to yet another aspect of the present invention, a system isprovided for calibrating an SEM. The system comprises an electron beamfor inducing a current in a reference sample feature, and a probe formeasuring an electrical characteristic of the reference sample featureto obtain an electrical reference measurement. The probe, for example,may comprise a current sensor for measuring the e-beam induced currentas well as a voltage sensor for measuring the voltage across thereference feature. The system further provides an electron beam toobtain an optical measurement of the reference feature, and a processoror other device adapted to correlate the optical and electricalreference measurements to obtain a reference sample feature CD. Theelectron beam used for the optical measurement of the reference samplefeature may be the SEM being calibrated, thus allowing simultaneouselectrical and optical measurement of the reference feature. The opticaland electrical measurements, however, may be done separately. Inaddition, the processor may perform calculation of calibrationmagnification or scaling coefficients, stochastics, neural networks,artificial intelligence, data fusion techniques, and/or trending incorrelating the optical and electrical reference feature measurements.The system can then be adapted to measure a workpiece feature, andcorrelate the reference sample CD with the workpiece measurement toobtain a workpiece feature CD.

In accordance with still another aspect of the invention, there isprovided a system for calibrating a scanning electron microscope,comprising a reference sample having a reference sample feature, anelectron beam adapted to induce a current in the reference samplefeature, and a probe in electrical communication with the referencesample feature and providing an electrical measurement of the referencesample feature. The probe may comprise current and voltage sensors, fordetermining the resistance of the reference feature, and a physicaldimension thereof. Also provided is a SEM adapted to provide an opticalmeasurement of the reference sample feature, and a processor or othermeans for correlating the optical and electrical measurements of thereference sample feature, whereby a reference feature CD is obtained.

The calibration system accounts for degradation in a reference sampleassociated with repeated usage in a SEM, and further allows trendinganalysis of the reference sample degradation. Further, the systemprovides for reduction in the errors associated with initial estimatesof the actual reference sample feature size, via an e-beam inducedcurrent measurement of the feature. Another aspect of the inventionprovides means for correlating the reference feature CD with a workpiecefeature measurement, whereby a workpiece feature CD is obtained.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrative examplesof the invention. These examples are indicative, however, of but a fewof the various ways in which the principles of the invention may beemployed. Other objects, advantages and novel features of the inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a method for calibrating ascanning electron microscope in accordance with the present invention;

FIG. 2 is a schematic diagram illustrating a conventional scanningelectron microscope;

FIG. 3 is a schematic diagram illustrating a conventional system forcalibrating a scanning electron microscope;

FIG. 4 is a schematic diagram illustrating a system and method forcalibrating a scanning electron microscope in accordance with theinvention;

FIG. 5 is a schematic diagram illustrating the system and method forcalibrating a scanning electron microscope of FIG. 4;

FIG. 6a is a plan view of an exemplary reference sample which may beused in the methods and systems of the present invention;

FIG. 6b is a sectional side elevation view of the exemplary referencesample taken along line 6 b—6 b of FIG. 6a;

FIG. 7 is a plan view of another exemplary reference sample which may beused in the methods and systems of the present invention;

FIG. 8 is a plan view of still another exemplary reference sample whichmay be used in the methods and systems of the present invention; and

FIG. 9 is a plan view of yet another exemplary reference sample whichmay be used in the methods and systems of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with reference to thedrawings, wherein like reference numerals are used to refer to likeelements throughout. FIG. 1 illustrates a method 2 for calibrating ascanning electron microscope. An e-beam is provided to a referencesample feature at step 4. A current is induced in the reference samplefeature by the e-beam. At step 6, an electrical characteristic of areference sample feature is measured, after which an SEM measurement ismade of the reference feature at step 8. The electrical and SEMreference feature measurements are correlated at step 10 to obtain acritical dimension (CD) for the reference feature. A feature of intereston a workpiece is then measured at step 12 using the SEM, and aworkpiece feature CD is obtained at step 14 by correlating the workpieceSEM measurement with the reference feature CD.

The method of FIG. 1 reduces or eliminates the effects of referencesample degradation on the CD measurement of the workpiece, since theelectrical measurement of the reference sample is used to correlate theSEM reference sample measurement. In particular, charge accumulation andcontamination deposition associated with repeated usage of a referencesample in a SEM system, can be accounted for because the electricalmeasurement of the reference is effected differently by thisdegradation, than is the SEM measurement thereof. Thus, a user mayperform trending analysis to determine the extent of a referencesample's degradation, and calibrate out the effects associated therewiththrough the correlation of the electrical and SEM referencemeasurements.

The electrical measurement and correlation also eliminate the errorsassociated with estimating the actual size of a reference feature.Moreover, temperature effects may be accounted for in the correlation ofthe electrical and optical measurements of the reference feature. Thecorrelation of the electrical and optical (SEM) measurements of areference sample feature may comprise various mathematical algorithms,such as for example, determining a scaling coefficient, stochastics,neural networks, artificial intelligence, data fusion techniques, andthe like.

The aspects of the present invention will be further illustratedhereinafter, in comparison with convention methods and systems, whichare discussed to provide context for the invention. In particular, FIG.2 illustrates a CD-SEM system 20 including a chamber 24 for housing awafer 26. An electron beam 28 is directed from an electromagnetic lens30 toward the wafer 26. The electron beam 28 is created from highvoltage supplied by a power supply 32 associated with a beam generatingsystem 34 which includes an emission element 34 a. Various directing,focusing, and scanning elements (not shown) in the beam generatingsystem 34 guide the electron beam 28 from the emission element 34 a tothe electromagnetic lens 30. The electron beam particles may beaccelerated to energies from about 500 eV to 40 Kev.

When the electron beam 28 strikes the surface of the wafer 26, electronsand x-rays are emitted which are detected by a detector 36 and areprovided to a detection system 38. The detection system 38 providessignals to a processing system 44 for performing conventional criticaldimension measurements and signal analysis, for example, to determinethe width of a line or other feature of interest on the wafer 26.

Electrons which are emitted from the surface of the wafer 26 which aremost useful for critical dimension imaging are known as secondary orsecondary electrons and provide a substantial amount of the signalcurrent received by the detector 36. A critical dimension image may alsobe directed to a display 40 by the processing system 44. The processingsystem 44, in addition to analyzing data received by the detectionsystem 38, synchronizes the scanning of the display 40 with electronbeam scanning of the wafer 26 to provide the image. Contrast of thedisplayed image is related to variations in the flux of electronsarriving at the detector 36 and is related to the yield of emittedelectrons from the surface of the wafer 26 to the incident electronsfrom the electron beam 28.

The detection system 38 receives the electron emissions from the surfaceof the wafer 26 via the detector 36 and preferably digitizes theinformation for the processing system 44. In addition, the detectionsystem 38 may also provide filtering or other signal processing of thereceived signal. The processing system 44 provides critical dimensioninformation to the display 40 and/or stores information in a memory 46.

A processor (not shown) is included in the processing system 44 forcontrolling the beam generating system 34, providing critical dimensionmeasurements, and for performing signal analysis. It is to beappreciated that a plurality of processors and/or processing systems maybe included as part of and/or external to the CD-SEM system 20. Theprocessor in the processing system 44 is programmed to control andoperate the various components within the CD-SEM system 20 in order tocarry out the various functions associated with the measurement of thewafer 26. The processor may be any of a plurality of processors, such asthe AMD Athlon, K6 or other type architecture processors.

A memory 46 is also included in the system 20. The memory 46 isoperatively coupled to the processing system 44 and serves to storeprogram code executed by the processor for carrying out operatingfunctions of the system 20 as described herein. The memory 46 alsoserves as a storage medium for temporarily storing information such ascalibration data, critical dimension data, statistical data, and otherdata. The power supply 32 also provides operating power to the CD-SEMsystem 20 along with providing a high voltage to the beam generatingsystem 34.

Referring now to FIG. 3, a conventional SEM calibration is shown,wherein a reference sample 50 having a reference feature 52 is locatedon a stage 54 and measured by the SEM system 20. The electron beam 28 isdirected onto the feature 52, and the detector 36 senses the secondaryelectrons 56, in order to obtain an optical or SEM measurement of thefeature 52 as the stage 54 is displaced relative to the lens 30 in oneor more directions perpendicular to the electron beam 28.

Where a physical characteristic of the feature 52, such as for example,the width of a conductor line is of interest, the sample 50 may bescanned via movement of the stage 54, whereby a change in the secondaryelectrons 56 can be determined and a distance (e.g., line width)calculated. Where the line width of the reference feature 52 is known,the reference feature measurement may be used to calibrate the SEM priorto its use in measuring features of a workpiece, such as a semiconductorwafer 26. However, where the dimensions or other characteristics of thereference sample feature 52 are not preceisely know, or change, due tocontamination, charge buildup, heat, or other causes, the conventionalcalibration method of FIG. 3 will not prevent errors in measuringworkpiece feature CDs.

Referring now to FIG. 4, a system and method for calibrating a scanningelectron microscope are illustrated and described in accordance with thepresent invention. The system 100 comprises a beam generator 134 havingan emission element 134 a which provides an electron beam 128 through alens 130 to a reference sample 150 and/or a workpiece 126, which aremounted on a stage 154 within a chamber 124, as will be discussed ingreater detail infra. Secondary electrons 156 are sensed by a detector136 which is operatively coupled to a detection system 138. A processingsystem 144 is provided which receives information from the detectingsystem 138. The processing system 144 provides critical dimensioninformation to a display 140 and/or stores information in a memory 146.A processor (not shown) is included in the processing system 144 forcontrolling the beam generating system 134, providing critical dimensionmeasurements, and for performing signal analysis. A plurality ofprocessors and/or processing systems may be included as part of and/orexternal to the CD-SEM system 100 for performing signal analysis andother operations associated with performing the measurement andcalibration methods according to the invention.

The processor in the processing system 144 is programmed to control andoperate the various components within the CD-SEM system 100 in order tocarry out the various functions described herein. The processor may beany of a plurality of processors, for example, such as the AMD Athlon,K6 or other type architecture processors. The memory 146 is operativelycoupled to the processing system 144 and serves to store program codeexecuted by the processor for carrying out operating functions of thesystem 100 as described herein. The memory 146 also serves as a storagemedium for temporarily storing information such as calibration data,critical dimension data, statistical data, and other data which may beemployed in carrying out the present invention. The power supply 132also provides operating power to the CD-SEM system 100 along withproviding a high voltage to the beam generating system 134. Any suitablepower supply (e.g., linear, switching) may be employed to carry out thepresent invention.

An electrical probe 160 is connected to the processing system 144 andcomprises electrical sensing circuitry or devices (not shown) formeasuring an electrical characteristic associated with one or morefeatures 162 a, 162 b, etc., (hereinafter referred to as 162) on thereference sample 150 via one or more lead wires 164. The stage 154 isadapted to move horizontally with respect to the vertical electron beam128 to effect a scanning of the workpiece 126 and/or the referencesample 150 by the beam 128. The electron beam 128 induces current flowin one or more of the features 162 of the reference sample 150. Theprobe 160 may comprise voltage and current sensors to measure thecurrent flow induced by the electron beam 128 in one or more of thereference features 162, as well as a voltage sensor adapted to sense thevoltage across one or more of the reference features 162. From thesensed current and voltage measurements, the resistance of the referencefeatures 162 may be obtained, by the probe 160 and/or the processingsystem 144. Many other electrical characteristics of sample features 162may be measured by the probe 160, such as for example, capacitance,inductance, and the like, using the sensed current induced by the beam128. Further, although two lead wires 164 are illustrated in FIG. 4,additional wires 164 may be provided as desired to measure electricalcharacteristics of the reference sample 150 and one or more of thefeatures 162 thereon.

In addition to the programs and instructions for carrying out SEMmeasurements, the processing system 144 is adapted to control themeasurement functions of the electrical probe 160 and to receive andanalyze data (not shown) from the probe 160 relating to measuredelectrical characteristics of the reference sample 150. In this regard,the processing system 144 and/or the memory 146 may include programs andinstructions for performing various mathematical algorithms, such as forexample, computation of calibration or scaling factors, stochastics,neural networks, artificial intelligence, data fusion techniques, andthe like. These algorithms may be advantageously used to correlateelectrical feature measurement data from the probe 160 with SEMmeasurement data from the detection system 138 in order to calibrate theSEM system 100 as described in greater detail infra. In particular, theprocessing system 144 may be used to correlate electrical and SEMmeasurements of one or more reference features 162 in order to obtain ordetermine a reference feature CD, and to correlate a reference featureCD with a workpiece feature SEM measurement in order to obtain ordetermine a workpiece feature CD, in accordance with one aspect of thepresent invention. The correlation of an electrical and an opticalmeasurement reduces or eliminates prior calibration errors associatedwith charge buildup, corrosion, and other degradation of a referencesample resulting from repeated usage in a SEM, temperature change, andthe like.

According to one aspect of the invention, the probe 160 and lead wires164 are employed to measure an electrical characteristic of a feature,such as feature 162 a, on the reference sample 150. The stage 154 may bepositioned as shown in FIG. 4 and an e-beam 128 is generated by the beamgenerating system 134, whereby the system 100 may perform an electricalmeasurement of the feature 162 a. The electrical characteristicmeasurement may be advantageously performed in the presence of thee-beam 128, so that a current (not shown) is induced by the beam 128 inthe reference feature 162. An electrical characteristic, for example,resistance, of a feature 162 a is measured by the probe 160 via leadwires 164. This may be accomplished, for example, by probe 160 measuringthe induced current flow and the voltage across the lead wires 164, andthe probe and/or the processing system determining the featureresistance based on the voltage and current measurements.

With the stage 154 positioned as shown in FIG. 4 and a beam 128generated by the beam generating system 134, the system 100 may performan SEM measurement of the feature 162 a. The SEM measurement obtainsoptical information about the reference feature, and may be performedsimultaneously or contemporaneously with the electrical characteristicmeasurement. This SEM measurement may be referred to as an opticalfeature measurement, while the electrical characteristic measurement maybe referred to as an electrical feature measurement.

The processing system 144 receives the electrical measurement data orinformation (not shown) from the electrical probe 160, and receives theSEM optical measurement data or information from the detection system138. Thereafter, the processing system 144 correlates the electrical andoptical measurements of the reference sample feature 162 a, via analgorithm such as for example, computation of a calibration or scalingfactor, stochastics, neural networks, artificial intelligence, datafusion techniques, and the like, to obtain a reference sample feature CD(not shown) for the feature 162 a. This correlation advantageouslyprovides for calibration of the SEM measurement, and may includecompensation for inaccuracies in estimation of a reference feature size,and corrosion, charge accumulation, temperature, and other deleteriousdegradation of a reference sample feature 162. The use of electricalreference feature measurement in conjunction with SEM measurement of thefeature 162, therefore, ensures better calibration of the SEM system 100and accuracy of subsequent SEM measurements of a workpiece feature byvirtue of the correlation.

As an example, the electrical reference feature measurement may be avalue for the resistance of a conductor line feature 162 a on areference sample 150. Where the length and height of the conductor lineand the conductivity of the feature material are known, the resistancemeasurement may be used by the processing system 144 to determine theline width of the feature 162 a. The SEM optical measurement of thefeature 162 a may be of the conductor line width. The correlationalgorithm may, for example, determine a calibration factor K (notshown), which is the fraction of the electrical reference featuremeasurement divided by the optical or SEM reference feature measurement.Where, for example, the electrical and optical reference featuremeasurements were 1.0 and 0.8 microns, respectively, the calibrationfraction in this example would be 1.25. Subsequent (or even prior) SEMmeasurements of workpiece features may be scaled by (e.g., correlatedwith) the calibration factor K to determine a CD for the workpiecefeature measured with the system 100. The invention thus provides foradjustment based on an actual electrical measurement of a referencefeature whose dimension was estimated in prior systems.

In addition, where the electrical and optical reference featuremeasurements are taken periodically (e.g., daily, weekly, etc.),trending analysis can be performed by the processing system 144, wherebythe degradation of a reference sample feature 162 may be determined. Asan example, trending may show that the electrical measurement of areference feature remains fairly constant over a period of time, whilethe optical measurement of the same reference feature decreases (orincreases) over the same timer period. This may be used, for example, todetermine that a new reference sample is needed, or that the SEM system100 performance is worsening. Alternatively, the trending may show thata reference feature is changing over time, or with temperature change,etc.

In addition to trending analysis, the processing system 144 may furtherperform data fusion analysis, wherein the electrical and opticalmeasurements are analyzed to determine other variables affecting systemperformance. In this context, data fusion is algorithmic processing ofmeasurement data or information to compensate for the inherentfragmentation of information because a particular phenomena may not beobserved directly using a single sensing element or measurement. Inother words, the data fusion architecture provides a suitable frameworkto facilitate condensing, combining, evaluating and interpreting theavailable optical and electrical measurement data or information in thecontext of the particular application, such as an SEM system 100.

It will be appreciated that the degradation of a reference samplefeature 162 may affect the optical and electrical measurements thereofdifferently, and that the correlation of electrical and optical (SEM)measurements of a reference sample feature 162 provides for improvedcalibration capabilities within the present invention. Moreover, it willbe recognized that many algorithms and correlation techniques areavailable to compensate for reference sample feature degradation andother errors in the system 100, for example, computing calibrationscaling factors, stochastics, neural networks, artificial intelligence,data fusion techniques, mathematical prediction/correction techniques,and the like.

Referring also to FIG. 5, once the reference feature CD has beenobtained or determined, the stage 154 may positioned such that theworkpiece 126 is beneath the electron beam 128. Thereafter, the system100 performs an SEM scan of one or more features of interest (not shown)on the workpiece 126, with workpiece feature measurement data orinformation being provided to the processing system 144 via thedetection system 138. The processing system 144 may then correlate theoptical workpiece feature measurement with the reference feature CD inorder to obtain or determine a workpiece feature CD. As with thecorrelation of the optical and electrical reference sample measurementsdiscussed supra, the correlation of the reference feature CD with theSEM or optical workpiece feature measurement by the processing system144 may comprise, for example, stochastics, neural networks, artificialintelligence, data fusion techniques, mathematical prediction/correctiontechniques, and the like.

It will be appreciated that the workpiece feature measurement may beperformed prior to the reference sample measurements, and further thatthe reference feature measurements (electrical and optical) may beperformed in any order or simultaneously. Furthermore, the measurementsneed not be performed contemporaneously, since the correlationalgorithms and trending analysis may account for the time the variousmeasurements are made. In this way, a workpiece CD may be obtained for aworkpiece measured a week prior to the calibration reference featuremeasurements used to correlate the workpiece measurements, etc.

In the exemplary correlation discussed supra, a calibration factor K wasdetermined from the correlation of the optical and electrical referencefeature measurements. In this example, a workpiece feature CD may beobtained by scaling the workpiece feature measurement by the calibrationfactor K. In this regard, a workpiece feature measurement of 8 micronsmay be multiplied by the calibration factor K (1.25), to obtain aworkpiece feature CD of 10 microns. The system and method of the presentinvention, thus account for degradation of a reference sample, andcalibrate out some or all of the errors caused by this degradation. Thiswas not possible in the prior systems, wherein the reference sample wasonly measured optically.

Referring also to FIGS. 6a and 6 b, an exemplary reference sample 200 isillustrated, having lead wires 164 for electrical connection with anelectrical probe 160 as illustrated in FIGS. 4 and 5. The referencesample 200 comprises a conductor line reference feature 202 extendingbetween connection points 204 and 206 on a substrate 208, and having awidth W and a height H. The reference feature line 202 extends a lengthL between two measurement pads 212. In electrically measuring thereference feature 202, the voltage across the length L of line referencefeature 202, and the induced current there through may be measured bythe probe 160. The resistance of the reference feature line 202 may becomputed therefrom. Knowing the conductivity of the reference featureline material and the length L and the height H of the referencefeature, the line width W may be calculated by the processing system144. An optical (SEM) measurement may be taken of the line width W alonga scan line 210, which can be correlated with the calculated line width(based on the electrical measurement of the feature line 202), to obtaina reference feature CD (not shown) for the feature 202. The referencefeature CD may then be used to correlate optical SEM workpiece featuremeasurements in order to determine workpiece feature CDs.

Referring also to FIGS. 7, 8, and 9, other exemplary reference samples250, 300, and 350, respectively, are illustrated. In FIG. 7, sample 250has a three section conductor line feature 252, having lengths L1, L2,and L3, respectively. Connectors 254 and 256 are provided on the sameside of the sample 250 for connection to a probe 160 using lead wires164. Additional measurement pads 212 are provided near the connectors254 and 256, for example, to provide voltage measurement connections tothe sample 250. The reference feature line 252 is fabricated on asubstrate 258 of known height and material, to enable a determination ofthe line width W based on a measurement by a probe 160 of an electricalcharacteristic of the line 252, such as current, voltage, resistance,etc. This electrical measurement can then be correlated with an SEMmeasurement along scan line 210 of the width W of the reference feature252 to obtain a reference feature CD.

In FIG. 8, reference sample 300 comprises several reference features 302a, 302 b, and 306 c, having widths W1, W2, and W3, respectively on asubstrate 308. Each of the reference features 302 can be measuredelectrically using a probe 160, connectors 304 and 306, measurement pads212, and lead wires 164, as discussed supra. The electrical measurementcan then be used to calculate the width W1, W2, and/or W3 of thereference features 302 a, 302 b, and 306 c, respectively, where theheight, conductivity, and length of the conductive paths there throughbetween the connectors 304 and 306 and/or the pads 212 are known.Optical measurements of the reference features 302 a, 302 b, and/or 306c, taken along a scan line 210 may then be correlated with theelectrical measurements thereof to obtain one or more reference featureCDs in the manner discussed above.

Referring now to FIG. 9, a reference sample 350 is illustrated havingthree reference feature lines 352 a, 352 b, and 356 c of differentwidths (not shown) on a substrate 358. One common connector 354 andindividual connectors 360, 362, and 364 are provided for connection to aprobe 160 via lead wires 164. Measurement pads 212 are also provided forconnection to individual reference feature lines 352 a, 352 b, and 356c. Electrical measurements of the reference feature lines 352 can thenbe correlated with optical SEM measurements taken along a scan line 210in the manner discussed above. Many different reference samples may beused in the methods and systems of the present invention, and are deemedto be within the scope thereof.

Although the invention has been shown and described with respect to acertain embodiments, it will be appreciated that equivalent alterationsand modifications will occur to others skilled in the art upon thereading and understanding of this specification and the annexeddrawings. In particular regard to the various functions performed by theabove described components (assemblies, devices, circuits, systems,etc.), the terms (including a reference to a “means”) used to describesuch components are intended to correspond, unless otherwise indicated,to any component which performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure, which performs thefunction in the herein illustrated exemplary embodiments of theinvention. In this regard, it will also be recognized that the inventionincludes a system for performing the steps of the various methods of theinvention.

In addition, while a particular feature of the invention may have beendisclosed with respect to only one of several embodiments, such featuremay be combined with one or more other features of the other embodimentsas may be desired and advantageous for any given or particularapplication. Furthermore, to the extent that the terms “includes”,“including”, “has”, “having”, and variants thereof are used in eitherthe detailed description or the claims, these terms are intended to beinclusive in a manner similar to the term “comprising.”

What is claimed is:
 1. A method for calibrating a scanning electronmicroscope, comprising: providing an electron beam to a feature of areference sample, whereby a current is induced in the reference samplefeature; measuring an electrical characteristic of the reference samplefeature to obtain an electrical reference measurement; measuring thereference sample feature using an electron beam to obtain an opticalreference measurement; correlating the optical and electrical referencemeasurements to obtain a reference sample feature CD; measuring afeature of a workpiece using an electron beam to obtain an opticalworkpiece measurement; and correlating the reference sample feature CDand the optical workpiece measurement to obtain a workpiece feature CD.2. The method of claim 1, wherein correlating the optical and electricalreference measurements comprises executing an algorithm using at leastone of a scaling coefficient, stochastics, neural networks, artificialintelligence, data fusion techniques, and trending.
 3. The method ofclaim 2, wherein the reference sample feature is the width of a line,and the electrical characteristic is resistance.
 4. The method of claim3, wherein measuring an electrical characteristic of the referencesample feature is before measuring the feature of the workpiece using anelectron beam.
 5. The method of claim 1, wherein correlating thereference sample feature CD and the optical workpiece measurementcomprises executing an algorithm using at least one of a scalingcoefficient, stochastics, neural networks, artificial intelligence, datafusion techniques, and trending.
 6. The method of claim 5, wherein thereference sample feature is the width of a line, and the electricalcharacteristic is resistance.
 7. The method of claim 6, whereinmeasuring an electrical characteristic of the reference sample featureis before measuring the feature of the workpiece using an electron beam.8. The method of claim 1, wherein the reference sample feature is thewidth of a line, and the electrical characteristic is resistance.
 9. Amethod for calibrating a scanning electron microscope, comprising:providing an electron beam to a feature of a reference sample, whereby acurrent is induced in the reference sample feature; measuring anelectrical characteristic of the reference sample feature to obtain anelectrical reference measurement; measuring the feature of the referencesample using an electron beam to obtain an optical referencemeasurement; and correlating the optical and electrical referencemeasurements to obtain a reference sample feature CD.
 10. The method ofclaim 9, further comprising correlating the reference sample feature CDwith an optical workpiece measurement to obtain a workpiece feature CD.11. A system for calibrating a scanning electron microscope, comprising:means for providing an electron beam to a feature of a reference sample,whereby a current is induced in the reference sample feature; means formeasuring an electrical characteristic of the reference sample featureto obtain an electrical reference measurement; means for measuring thereference sample feature using an electron beam to obtain an opticalreference measurement; means for correlating the optical and electricalreference measurements to obtain a reference sample feature CD; meansfor measuring a feature of a workpiece using an electron beam to obtainan optical workpiece measurement; and means for correlating thereference sample feature CD and the optical workpiece measurement toobtain a workpiece feature CD.
 12. The system of claim 11, wherein themeans for measuring an electrical characteristic comprises an electricalprobe in electrical communication with the reference sample.
 13. Thesystem of claim 12, wherein the electrical probe comprises a currentsensor adapted to measure the electron beam induced current in thereference sample, and a voltage sensor for detecting the voltage acrossthe reference sample.
 14. The system of claim 13, wherein the electricalcharacteristic of the feature of the reference sample is resistance. 15.The system of claim 14, wherein the means for correlating the opticaland electrical reference measurements comprises a processor adapted toexecute at least one of a scaling coefficient, stochastics, neuralnetworks, artificial intelligence, data fusion techniques, and trending.16. The system of claim 11, wherein the means for correlating theoptical and electrical reference measurements comprises a processoradapted to execute at least one of a scaling coefficient, stochastics,neural networks, artificial intelligence, data fusion techniques, andtrending.
 17. A system for calibrating a scanning electron microscope,comprising: a reference sample having a reference sample feature; anelectron beam adapted to induce a current in the reference samplefeature; a probe in electrical communication with the reference samplefeature and providing an electrical measurement of the reference samplefeature; a SEM adapted to provide an optical measurement of thereference sample feature; and means for correlating the optical andelectrical measurements of the reference sample feature, whereby areference feature CD is obtained.
 18. The system of claim 17, fuirthercomprising means for correlating the reference feature CD with aworkpiece feature measurement, whereby a workpiece feature CD isobtained.
 19. The system of claim 18, wherein the means for correlatingthe reference feature CD with a workpiece feature measurement comprisesa processor adapted to execute at least one of a scaling coefficient,stochastics, neural networks, artificial intelligence, data fusiontechniques, and trending.
 20. The system of claim 19, wherein theelectrical measurement of the feature of the reference sample comprisesmeasurement of the electron induced current in the reference samplefeature and the voltage across the reference sample feature, whereby theresistance of the reference sample feature may be determined.
 21. Thesystem of claim 20, wherein the probe comprises a voltage sensor adaptedto measure the voltage across the reference sample feature, and acurrent sensor for detecting the electron beam induced current throughthe reference sample feature.
 22. The system of claim 17, wherein themeans for correlating the optical and electrical measurements of thereference sample feature comprises a processor adapted to execute atleast one of a scaling coefficient, stochastics, neural networks,artificial intelligence, data fusion techniques, and trending.